A polyamic acid solution is used in various applications as a polyimide precursor solution. In general, a polyamic acid solution may be suitably prepared by reacting substantially equimolar amounts of a diamine and a tetracarboxylic dianhydride in a solvent at a low temperature for suppressing an excessive imidization reaction. However, it is difficult to produce a polyamic acid solution having a high concentration by this process because a polyamic acid is readily polymerized, leading to higher viscosity of the solution.
Patent Document 1 discloses a process for producing a polyamic acid solution composition in which substantially equimolar amounts of a biphenyltetracarboxylic dianhydride and an aromatic diamine are reacted at a reaction temperature of 100° C. or lower in an organic polar solvent containing about 0.5 mole to about 40 mole of water per mole of the biphenyltetracarboxylic dianhydride to give a homogeneous reaction solution, and then free water is removed from the reaction solution to lower the content of free water in the reaction solution as measured by nuclear magnetic resonance spectrometry to 0.5 wt % or less. However, because the reaction of an acid anhydride group and an amino group and the reaction of an acid anhydride group and water proceed competitively in this process, it is difficult to control the reaction, and therefore it is not easy to control a molecular weight of a polyamic acid obtained reproducibly and reliably. Furthermore, in Examples of Patent Document 1, with regard to low molecular weight of the polyamic acid, the polyamic acid solution having a polyamic acid logarithmic viscosity of 0.43 was obtained (Example 13). With regard to high concentration of the polyamic acid solution, the polyamic acid solution having a polymer concentration of 26.0 wt % was obtained (Example 9). Nevertheless, it is still difficult to produce a polyamic acid solution having a higher concentration and a lower molecular weight of the polyamic acid. In addition, water must be removed from the reaction solution after the completion of the reaction in this process, which requires a complicated step.
Patent Documents 2 and 3 disclose a process for producing a polyamic acid solution by polymerizing a tetracarboxylic acid component and a diamine component, for example, by polymerizing substantially equimolar amounts of an acid component and an amine component in an organic polar solvent containing a small amount of water, or alternatively, by reacting an amine component with a molar excess of an acid component to prepare an intermediate such as an oligomer, and then adding an amine component thereto so that the molar amount of the amine component is substantially equal to the molar amount of the acid component, followed by further reacting them to prepare a polyamic acid. However, Patent Documents 2 and 3 merely teach that the amount of water contained in the solvent is about 10 wt % or less. Patent Documents 2 and 3 do not present Example in which a polyamic acid solution is produced by reacting an amine component with a molar excess of an acid component, and then adding an amine component thereto so that the molar amount of the amine component is substantially equal to the molar amount of the acid component, followed by further reacting them. In Comparative Example 6 of Patent Document 3, a polyamic acid solution having a polyamic acid logarithmic viscosity of 0.39 was obtained, but the polyamic acid solution had a polymer concentration of 22.3 wt %. Thus, there is the need for a polyamic acid solution having a higher concentration and a lower molecular weight of the polyamic acid. In addition, the polyamic acid solution of Comparative Example 6 of Patent Document 3 did not have adequate solution stability.
Patent Document 1: Japanese Laid-open Patent Publication No. 1982-131248.
Patent Document 2: Japanese Laid-open Patent Publication No. 1984-164328.
Patent Document 3: Japanese Laid-open Patent Publication No. 1984-204619.